Precipitation hardening martensitic stainless steel, turbine component formed of said martensitic stainless steel, and turbine including said turbine component

ABSTRACT

It is an objective of the invention to provide a precipitation-hardening martensitic stainless steel having a far better balance between a high mechanical strength and a high toughness than conventional ones as well as having good corrosion resistance properties. There is provided a precipitation-hardening martensitic stainless steel throughout which precipitates of intermetallic compounds are dispersed, the martensitic stainless steel including: 0.1 mass % or less of C; 11 to 13 mass % of Cr; 7.5 to 11 mass % of Ni; 0.9 to 1.7 mass % of Al; 0.85 to 1.35 mass % of Mo; 1.75 to 2.75 mass % of W; and the balance including Fe and inevitable impurities, in which “[Mo content]+0.5×[W content]” is from 1.9 mass % to 2.5 mass %, and “[Mo content]/[W content]” is from 0.4 to 0.6.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2013-231943 filed on Nov. 8, 2013, the content of which ishereby incorporated by reference into this application.

1. FIELD OF THE INVENTION

The present invention relates to steels having high mechanicalproperties and a high corrosion resistance, and particularly to aprecipitation-hardening martensitic stainless steel. The invention alsoparticularly relates to a turbine component formed of such aprecipitation-hardening martensitic stainless steel of the invention,and a turbine including such a turbine component of the invention.

2. DESCRIPTION OF RELATED ART

Because of the recent trend toward the conservation of energies (such asfossil fuel energy) and the global environment conservation (such assuppression of CO₂ gas emission), a strong demand exists to increase theefficiencies of apparatuses (such as steam turbines) used in thermalpower plants. An effective means to improve the efficiency of steamturbines is to increase length of the blades (such as long blades) ofthe turbine. By using longer turbine long blades, more steam energy canbe converted into rotational energy of a turbine. Such an increasedconversion efficiency has an additional effect of reducing the number ofturbine casings, thereby leading to a reduction in construction time andcost.

Currently, long blades of steam turbines in ultra super critical (USC)power plants are formed mainly of martensitic stainless steels. Aproblem with such a steam turbine long blade is that the longer a steamturbine long blade is, the much stronger centrifugal force the bladereceives (because, generally, the centrifugal force is proportional tothe product of the mass of the blade and the rotational radius of theturbine (the length of the turbine blade)). Therefore, there is a strongneed for steam turbine long blade materials having a higher mechanicalstrength than conventional materials. Such long blade materials alsorequire a high toughness in order to prevent sudden rupture.

Various structural materials having both a high mechanical strength anda high toughness have been proposed. For example, JP 2005-194626 Adiscloses a precipitation-hardening martensitic stainless steelincluding: 12.25 to 14.25 wt. % of Cr, 7.5 to 8.5 wt. % of Ni; 1.0 to2.5 wt. % of Mo; 0.05 wt. % or less of C; 0.2 wt. % or less of Si; 0.4wt. % or less of Mn; 0.03 wt. % or less of P; 0.005 wt. % or less of S;0.008 wt. % or less of N; 0.90 to 2.25 wt. % of Al; and the balancepractically Fe, wherein the total amount of Cr and Mo is from 14.25 to16.75 wt. %.

JP 2011-225913 A discloses a precipitation-hardening martensiticstainless steel including: 0.05 to 0.10 mass % of C, 12.0 to 13.0 mass %of Cr; 6.0 to 7.0 mass % of Ni; 1.0 to 2.0 mass % of Mo; 0.01 to 0.05mass % of Si; 0.06 to 1.0 mass % of Mn; 0.3 to 0.5 mass % of Nb; 0.3 to0.5 mass % of V; 1.5 to 2.5 mass % of Ti; 1.0 to 2.3 mass % of Al; andthe balance including Fe and inevitable impurities.

JP 2012-102638 A discloses a precipitation-hardening martensiticstainless steel including: 0.10 mass % or less of C, 13.0 to 15.0 mass %of Cr; 7.0 to 10.0 mass % of Ni; 2.0 to 3.0 mass % of Mo; 0.5 to 2.5mass % of Ti; 0.5 to 2.5 mass % of Al; 0.5 mass % or less of Si; 0.1 to1.0 mass % of Mn; and the balance including Fe and inevitableimpurities.

In spite of the growing worldwide responsibility towards globalenvironment conservation, the world's energy demand is continuing torise. In order to meet both of these conflicting demands, there is astrong need to further increase the efficiency of thermal power plants(in particular steam turbines). As already described, an effective wayto increase the efficiency of steam turbines is to increase the lengthof turbine long blades. A material used to form suchlonger-than-conventional turbine long blades needs to have both a highermechanical strength and a higher toughness than conventional martensiticstainless steels (such as the ones disclosed in JP 2005-194626 A, JP2011-225913 A and JP 2012-102638 A). In addition, steam turbine longblades are used in a harsh corrosive environment because they areexposed to a severe dry and wet cycle. Therefore, materials used forsteam turbine long blades also require a high corrosion resistance (suchas a high stress corrosion cracking (SCC) resistance).

Generally, there is a trade off between the mechanical strength andcorrosion resistance of steels. Common martensitic stainless steels havea high mechanical strength, but have a relatively poor corrosionresistance. In contrast, precipitation-hardening martensitic stainlesssteels, which contain a relatively large amount of Cr and a relativelysmall amount of C, are excellent in corrosion resistance, but arerelatively poor in mechanical strength.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an objective of the present invention toprovide a precipitation-hardening martensitic stainless steel having afar better balance between a high mechanical strength and a hightoughness than conventional ones as well as having good corrosionresistance properties (such as a high SCC resistance and a high pittingpotential). Another objective of the invention is to provide a turbinecomponent formed of such a precipitation-hardening martensitic stainlesssteel of the invention, and a turbine including such a turbine componentof the invention.

(I) According to one aspect of the present invention, there is provideda precipitation-hardening martensitic stainless steel throughout whichprecipitates of intermetallic compounds are dispersed, the martensiticstainless steel including: 0.1 mass % or less of C (carbon); 11 to 13mass % of Cr (chromium); 7.5 to 11 mass % of Ni (nickel); 0.9 to 1.7mass % of Al (aluminum); 0.85 to 1.35 mass % of Mo (molybdenum); 1.75 to2.75 mass % of W (tungsten); and the balance including Fe (iron) andinevitable impurities, in which “[content of the Mo]+0.5×[content of theW]” is from 1.9 mass % to 2.5 mass %, and “[content of the Mo]/[contentof the W]” is from 0.4 to 0.6.

In the above aspect (I) of the invention, the following modificationsand changes can be made.

i) The precipitation-hardening martensitic stainless steel furtherincludes 0.4 mass % or less of Ti (titanium).

ii) Part of the Ni is substituted by 3 mass % or less of Co (cobalt).

iii) The precipitation-hardening martensitic stainless steel furtherincludes one or both of Nb (niobium) and V (vanadium) in total amount of0.5 mass % or less.

iv) The precipitation-hardening martensitic stainless steel furtherincludes 0.1 mass % or less of Si (silicon) and/or 1 mass % or less ofMn (manganese).

v) The inevitable impurities include one or more of 0.5 mass % or lessof P (phosphorus), 0.5 mass % or less of S (sulfur), 0.1 mass % or lessof Sb (antimony), 0.1 mass % or less of Sn (tin), 0.1 mass % or less ofAs (arsenic), and 0.1 mass % or less of N (nitrogen).

vi) One of the intermetallic compounds is β-NiAl phase.

vii) The precipitation-hardening martensitic stainless steel is solutionheat treated at 850 to 950° C. followed by aging heat treatment at 450to 650° C.

(II) According to another aspect of the present invention, there isprovided a turbine component formed of the precipitation-hardeningmartensitic stainless steel of the invention.

(III) According to still another aspect of the present invention, thereis provided a turbine rotor including the turbine component of theinvention, in which the turbine component is a steam turbine long blade.

(IV) According to yet another aspect of the present invention, there isprovided a steam turbine including the turbine rotor of the invention.

(V) According to a further aspect of the present invention, there isprovided a thermal power plant including the steam turbine of theinvention.

Advantages of the Invention

According to the present invention, it is possible to provide aprecipitation-hardening martensitic stainless steel having a far betterbalance between a high mechanical strength and a high toughness thanconventional ones as well as having good corrosion resistance properties(such as a high SCC resistance and a high pitting potential). Alsopossible is to provide a turbine component formed of such aprecipitation-hardening martensitic stainless steel of the invention, aturbine including such a turbine component of the invention, and athermal power plant including such a turbine of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a perspective view of anexemplary steam turbine long blade formed of the invention's stainlesssteel;

FIG. 2 is a schematic illustration showing a longitudinal sectional viewof an example of a turbine according to the invention;

FIG. 3 is a system diagram of an example of a thermal power plantaccording to the invention;

FIG. 4 is a graph showing a relationship between a “[Mo content]/[Wcontent]” ratio and tensile strength; and

FIG. 5 is a graph showing a relationship between a “[Mo content]/[Wcontent]” ratio and impact energy absorption.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings. However, the invention isnot limited to the specific embodiments described below, but variouscombinations and modifications are possible without departing from thespirit and scope of the invention.

(Composition of Precipitation-Hardening Martensitic Stainless Steel)

The composition of the precipitation-hardening martensitic stainlesssteel of the invention will be described below.

C Component:

The C suppresses formation of δ-ferrite phase which has an adverseeffect on the mechanical properties and corrosion resistance of thestainless steel. Also, carbon forms a carbide with Cr or Ti, therebyprecipitation-hardening the steel. However, when the C content exceeds0.1 mass %, the toughness of the steel decreases due to excessiveprecipitation of the carbides; the corrosion resistance decreases due todecreased Cr concentration near the grain boundaries; and themartensitic transformation temperature lowers. Therefore, the C contentis preferably 0.1 mass % or less, more preferably 0.05 mass % or lessand even more preferably 0.025 mass % or less.

Cr Component:

The Cr forms a passivation film at the surface of the stainless steel,thereby improving the corrosion resistance. When the Cr content is lessthan 11 mass %, sufficient corrosion resistance cannot be achieved. Whenthe Cr content is more than 13 mass %, δ-ferrite phase is prone to form,thereby degrading the mechanical properties and corrosion resistance.Therefore, the Cr content is preferably from 11 to 13 mass %, morepreferably from 11.5 to 12.5 mass % and even more preferably from 11.75to 12.25 mass %.

Ni Component:

The Ni suppresses δ-ferrite phase formation and enhances the tensilestrength of the stainless steel by the dispersion/precipitationhardening effect of Ni-based intermetallic compounds (e.g., Ni—Alcompounds). The Ni also has an effect of increasing the quench hardeningproperties and the toughness of the stainless steel. These effects areinsufficient at Ni contents less than 7.5 mass %. At Ni contents morethan 11 mass %, some of austenite phase is retained and precipitates,thereby degrading the mechanical strength (such as tensile strength) ofthe steel. Accordingly, the Ni content is preferably from 7.5 to 11 mass%, more preferably from 8.5 to 10.5 mass % and even more preferably from9 to 10 mass %.

Al Component:

The Al, too, forms Ni—Al intermetallic compounds, thereby enhancing theprecipitation hardening effect on the steel. This effect is insufficientat Al contents less than 0.9 mass %. At Al contents more than 1.7 mass%, Ni—Al intermetallic compounds precipitate excessively and δ-ferritephase is prone to form, thereby degrading the steel characteristics.Accordingly, the Al content is preferably from 0.9 to 1.7 mass %, morepreferably from 1.1 to 1.5 mass % and even more preferably from 1.25 to1.4 mass %.

Mo Component:

The Mo improves the corrosion resistance of the steel and also increasesthe mechanical strength (by, for example, solid solution strengthening).These effects are insufficient at Mo contents less than 0.85 mass %. AtMo contents more than 1.35 mass %, δ-ferrite phase formation and/or theexcessive formation of Fe-based intermetallic compounds (such as Lavesphases) is promoted, thereby degrading the mechanical properties and/orcorrosion resistance of the steel. Accordingly, the Mo content ispreferably from 0.85 to 1.35 mass %, more preferably from 1 to 1.3 mass% and even more preferably from 1.1 to 1.2 mass %.

W Component:

The W, like the Mo, improves the corrosion resistance of the steel andalso increases the mechanical strength (by, for example, solid solutionstrengthening). These effects are insufficient at W contents less than1.75 mass %. At W contents more than 2.75 mass %, δ-ferrite phaseformation and/or the excessive formation of Fe-based intermetalliccompounds (such as Laves phases) is promoted, thereby degrading themechanical properties and/or corrosion resistance of the steel.Accordingly, the W content is preferably from 1.75 to 2.75 mass %, morepreferably from 2 to 2.5 mass % and even more preferably from 2.2 to 2.5mass %.

The compositional balance between the Mo and the W is the most importantparameter for the invention. The preferable Mo—W balance according theinvention is as follows: The weighted combined content expressed by “[Mcontent]+0.5×[W content]” is an important parameter and preferably from1.9 to 2.5 mass % and more preferably from 2 to 2.4 mass %. The Mo/Wcontent ratio is also important and preferably from 0.4 to 0.6. Byadjusting the Mo and W contents to this preferable range, aprecipitation-hardening martensitic stainless steel providing a moreexcellent balance among several important mechanical properties thanconventional martensitic stainless steels can be achieved. For example,a high mechanical strength (a tensile strength of 1550 MPa or more) anda high toughness (an impact energy absorption of 30 J or more) can beboth achieved.

Ti Component:

The Ti forms carbides and intermetallic compounds (e.g., Ni—Ti—Alcompounds), thereby precipitation-hardening the stainless steel. Ticarbides are preferentially formed over Cr carbides. As a result, theformation of Cr carbides is suppressed, thereby increasing the corrosionresistance of the stainless steel. The addition of Ti is not essentialfor the invention, but is preferable because it has some positiveeffects on the stainless steel of the invention. However, the additionof Ti exceeding 0.4 mass % causes the excessive precipitation ofintermetallic compounds and the formation of undesirable phases (such asσ-ferrite phase), thereby degrading the mechanical properties (such astoughness) of the stainless steel. Accordingly, the Ti content ispreferably 0.4 mass % or less, more preferably 0.35 mass % or less, andeven more preferably 0.3 mass % or less.

Co Component:

The Co suppresses the δ-ferrite phase formation. The Co also adjusts themartensitic transformation temperature, thereby increasing theuniformity of the martensite matrix. The addition of Co is not essentialfor the invention, but, preferably, the Ni is partially replaced by theCo because the Co has some positive effects on the stainless steel ofthe invention. In the case when the Co is added, the total content ofthe Ni and Co is preferably from 7.5 to 11 mass %. However, when the Cocontent exceeds 3 mass %, some of austenite phase is prone to beretained and the amount of the precipitation of Ni—Al intermetalliccompounds decreases, thereby degrading the mechanical strength (such astensile strength) of the steel. Therefore, the Co content is preferably3 mass % or less, and more preferably 2.8 mass % or less.

Nb Component:

The Nb forms precipitable carbides, thereby improving the mechanicalstrength of the stainless steel. The addition of Nb is not essential forthe invention, but is preferable because the Nb has some positiveeffects on the stainless steel. However, when the Nb content exceeds 0.5mass %, the δ-ferrite phase formation is promoted. Therefore, the Nbcontent is preferably 0.5 mass % or less, and more preferably 0.45 mass% or less.

V Component:

The V may be substituted for part or all of the Nb. In this case, it ispreferable that the total content of V and/or Nb is the same as thepreferable Nb content when added alone. Therefore, the total content ofNb and/or V is preferably 0.5 mass % or less, and more preferably 0.45mass % or less. Although the addition of V is not essential for theinvention, a combined addition of V and Nb has an effect of enhancingthe precipitation hardening.

Si Component:

The Si functions as a deoxidizing agent when melting the stainlesssteel. Even a small addition of Si is effective in providing suchdeoxidizing function. The addition of Si is not essential for theinvention, but is preferable because the Si has some positive effects onthe stainless steel. Si contents more than 1 mass % result in arelatively strong tendency to form δ-ferrite phase, thus deterioratingthe characteristics of the stainless steel. Accordingly, the Si contentis preferably 1 mass % or less, more preferably 0.5 mass % or less, andeven more preferably 0.25 mass % or less. When the stainless steel ismelted by vacuum carbon deoxidation or electroslag remelting, nointentional Si addition is required.

Mn Component:

The Mn functions as deoxidizing and desulfurizing agents when meltingthe stainless steel. Even a small addition of Mn is effective inproviding such deoxidizing and desulfurizing functions. The Mn also hasan effect of suppressing the δ-ferrite phase formation. The addition ofMn is not essential for the invention, but is preferable because the Mnhas some positive effects on the stainless steel. However, at Mncontents exceeding 1 mass %, some of austenite phase is prone to beretained. Accordingly, the Mn content is preferably 1 mass % or less,more preferably 0.5 mass % or less, and even more preferably 0.25 mass %or less. When melting the stainless steel by vacuum induction melting(VIM) or vacuum arc remelting (VAR), no intentional Mn addition isrequired.

Inevitable Impurities:

The term “inevitable impurity”, as used herein and in the appendedclaims, refers to an unintentionally contained material such as oneoriginally contained in a starting material and one contaminated duringmanufacture. Examples of inevitable impurities are P, S, Sb, Sn, As andN. The precipitation-hardening martensitic stainless steel of theinvention unavoidably contains one or more such inevitable impurities.

Reduction of the P and/or S improves the toughness of the stainlesssteel without sacrificing the mechanical strength; therefore, it ispreferable that each of the P and S contents is suppressed to as low aspossible. Each of the P and S contents is preferably 0.5 mass % or lessand more preferably 0.1 mass % or less in order to increase thetoughness.

Reduction of the Sb, Sn and/or As, too, improves the toughness.Therefore, it is also preferable that each of the Sb, Sn and As contentsis suppressed to as low as possible. Specifically, each of the Sb, Snand As contents is preferably 0.1 mass % or less and more preferably0.05 mass % or less.

The N has a strong affinity for the Al and the Ti and easily formsnitrides (such as AlN and TiN), thereby decreasing the toughness of thestainless steel. Such nitride formation has another adverse effect ofreducing the formation of precipitation strengthening intermetalliccompounds (such as Ni—Al and Ni—Ti—Al compounds), thereby reducing themechanical strength of the stainless steel. Therefore, the N content ispreferably suppressed to as low as possible. Specifically, the N contentis preferably 0.1 mass % or less and more preferably 0.05 mass % orless.

(Manufacturing Method)

Except for heat treatment, there is no particular limitation on themethod for manufacturing the precipitation-hardening martensiticstainless steel of the invention, but any conventional method may beused. The preferred heat treatment according to the invention will bedescribed below.

First, an un-heat-treated stainless steel is solution heat treated byheating the steel to a temperature from 800 to 1000° C. (more preferablyfrom 850 to 950° C.), maintaining it at this temperature, and thenquenching it. By this solution heat treatment, constituent chemicalelements (such as Ni, Al and Ti) of precipitable compounds are dissolvedin the austenitic matrix to form solid solutions during the heating, andthe austenitic matrix is then transformed to the martensitic matrix bythe quenching.

Preferably, the solution heat-treated steel is aging heat treated byheating it to a temperature from 450 to 650° C. (more preferably from500 to 600° C.), maintaining it at this temperature, and then cooling itslowly. By this aging heat treatment, intermetallic compounds (such asβ-NiAl phase) and carbides are formed and precipitated. By thesesolution and aging heat treatments, a precipitation-hardeningmartensitic stainless steel having a desirable fine structure, in whichfine precipitates are dispersed throughout a homogeneous martensitematrix, can be achieved.

More preferably, after the solution and aging heat treatments, a subzerotreatment is performed in order to reduce the retained austenite. Thesubzero treatment of the invention involves cooling a steel to atemperature lower than room temperature and maintaining it at thistemperature in order to transform any retained austenite to martensite.The subzero treatment of the invention involves, for example, cooling asteel to −70° C. or lower using a coolant (such as dry ice and liquidnitrogen) and an organic solvent (such as isopentane), and maintainingit at this temperature for 4 hours or longer.

When forming a turbine component from the invention's martensiticstainless steel, the component forming may be performed after carryingout all of the above heat treatments. Alternatively and preferably, thecomponent forming may be conducted at an intermediate process stagebetween the solution heat treatment and the aging heat treatment. Thisis because at such a process stage, any precipitable materials (such asNi—Al compounds) have not yet been precipitated, and therefore thestainless steel is easier to be deformed and handled. In the lattercase, the aging heat treatment can be carried out after the componentforming.

(Turbine Component)

Because the precipitation-hardening martensitic stainless steel of theinvention has both good mechanical properties and a good corrosionresistance, it can be advantageously applied to turbine components(e.g., steam turbine long blades of 50 inches or more and gas turbinecompressor blades). FIG. 1 is a schematic illustration showing aperspective view of an exemplary steam turbine long blade formed of theinvention's stainless steel. As illustrated in FIG. 1, a steam turbinelong blade 10 is of an axial entry type. The steam turbine long blade 10includes a blade profile section 11 (on which high-speed steam impinges)and a blade root section 12. In order to connect adjacent blades 10, astub 14 is formed at a central position of the profile section 11 and ashroud 15 is formed along the end edge of the profile section 11. Anerosion shield 13 is formed on a leading edge portion of the profilesection 11 in order to protect the profile section 11 from erosioncaused by impingement of high-speed steam containing liquid waterparticles. The erosion shield 13 is optional depending on the severityof the erosive environment. That is, the erosion shield 13 may notnecessarily be equipped when the erosion is not very severe, because theinvention's steel has a sufficient erosion resistance.

An example of the erosion shield 13 is a Co based alloy plate (e.g., astellite (registered trademark) plate). The stellite plate can be bondedto the profile section 11 by TIG welding, electron beam welding, brazingor the like. Preferably, after the bonding of the erosion shield 13, astress removal (SR) heat treatment is performed at 550 to 650° C. (morepreferably 570 to 630° C.) to remove residual stresses potentiallycausing cracks. Another way to protect the profile section 11 fromerosion is a surface hardening technique, which involves hardening thesurface of a leading edge portion of the profile section 11 by localheating using a high heat input laser or the like.

(Turbine)

FIG. 2 is a schematic illustration showing a longitudinal sectional viewof an example of a turbine according to the invention. As illustrated inFIG. 2, a low-pressure stage steam turbine 20 mainly includes a turbinerotor 21 rotatable by an operating fluid (steam) passing therethroughand a turbine casing 25 housing the turbine rotor 21. The turbine rotor21 includes a rotation shaft 22 and a plurality of axially spaced rotordisks 23 on the rotation shaft 22. Each rotor disk 23 has, on itscircumference, a plurality of circumferentially spaced radially orientedturbine long blades 10. The turbine long blades 10 are typically sodesigned as to increase in length with increasing distance downstream ofthe flow of the operating fluid. The turbine casing 25 includes aplurality of vanes 26, an operating fluid inlet 27, and an operatingfluid outlet 28. The vanes 26 are bonded to the inner wall of theturbine casing 25 in such a way that each vane 26 is positioned betweenadjacent turbine long blades 10.

(Thermal Power Plant)

FIG. 3 is a system diagram of an example of a thermal power plantaccording to the invention. As shown in FIG. 3, in a thermal power plant30, high-temperature, high-pressure steam (operating fluid) is producedin a boiler 31, generates mechanical power in a high-pressure stageturbine 32, and is reheated in the boiler 31. Then, the reheated steamgenerates an additional mechanical power in an intermediate-pressurestage turbine 33, and generates a yet additional mechanical power in alow-pressure stage turbine 20. The mechanical power generated in thesesteam turbines is converted into an electrical power by an electricgenerator 34. The steam exiting the low-pressure stage turbine 20 isintroduced into a steam condenser 35 to condense the steam back intowater, which is returned to the boiler 31.

Examples

The present invention will be described in more detail below by way ofexamples. However, the invention is not limited to the specific examplesbelow.

(Preparation of Inventive Stainless Steels 1 to 12 and ComparativeStainless Steels 1 to 17)

First, various steel ingots having different compositions were preparedby melting different sets of starting materials in a vacuumhigh-frequency induction melting furnace (5.0×10⁻³ Pa or lower) at atemperature of 1600° C. or higher. Each steel ingot was hot-forged intoa rectangular bar (100 mm wide, 30 mm thick, 1000 mm long) using a1000-ton forging machine and a 250-kgf hammer forging machine. Next, therectangular bar was further cut into a starting (un-heat-treated) samplebar (50 mm wide, 30 mm thick, 120 mm long).

The sample bars were subjected to the following heat treatments using abox furnace: First, each sample bar was solution heat treated bymaintaining the sample bar at 900° C. for 1 hour and then dipping itinto room temperature water (water-quenching). Then, the sample bar wasaging heat treated by maintaining it at 538° C. for 2 hours and thenair-cooling it in room temperature air. Any subzero treatment was notperformed.

Tables 1 to 5 show the results of the chemical composition analysis ofthe above-described stainless steel ingots. Although, except for the N,the contents of inevitable impurities (the P, S, Sb, Sn and As) are notshown in Tables, these inevitable impurities contents were within therange specified by the invention.

TABLE 1 Chemical Composition of Inventive Stainless Steels 1 to 6.(Unit: mass %) Inventive Inventive Inventive Inventive InventiveInventive Stainless Stainless Stainless Stainless Stainless StainlessSteel 1 Steel 2 Steel 3 Steel 4 Steel 5 Steel 6 C 0.01 0.01 0.01 0.010.03 0.01 Cr 12.08  11.08  12.90  12.14  12.02  12.10  Ni 9.51 9.52 9.497.61 10.91  9.44 Al 1.30 1.32 1.31 1.34 1.31 0.92 Mo 1.15 1.14 1.16 1.151.13 1.13 W 2.27 2.24 2.27 2.25 2.26 2.25 Ti — — — — — — Co — — — — — —Nb + V — — — — — — Si — — — — — — Mn — — — — — 0.15 N — — — — 0.01 —Fe + Balance Inevitable Impurities Mo + 0.5W 2.29 2.26 2.30 2.28 2.262.26 Mo/W 0.51 0.51 0.51 0.51 0.50 0.50 Note 1: Mark “—” means that theelement was not intentionally added or the element was below detectionlimit. Note 2: “Inevitable Impurities” are P, S, Sb, Sn and As.

TABLE 2 Chemical Composition of Inventive Stainless Steels 7 to 12.(Unit: mass %) Inventive Inventive Inventive Inventive InventiveInventive Stainless Stainless Stainless Stainless Stainless StainlessSteel 7 Steel 8 Steel 9 Steel 10 Steel 11 Steel 12 C 0.01 0.01 0.01 0.010.01 0.01 Cr 11.94  12.04  11.89  12.01  12.01  12.04  Ni 9.45 9.50 9.539.52 8.01 9.38 Al 1.63 1.28 1.30 1.31 1.28 1.33 Mo 1.14 1.01 1.24 1.131.14 1.11 W 2.25 2.02 2.44 2.23 2.22 2.22 Ti — — — 0.30 — — Co — — — —1.52 — Nb + V — — — — — 0.44 Si 0.23 — — — — — Mn — — — — — — N — — — —— — Fe + Balance Inevitable Impurities Mo + 0.5W 2.27 2.02 2.46 2.252.30 2.22 Mo/W 0.51 0.50 0.51 0.51 0.53 0.50 Note 1: Mark “—” means thatthe element was not intentionally added or the element was belowdetection limit. Note 2: “Inevitable Impurities” are P, S, Sb, Sn andAs.

TABLE 3 Chemical Composition of Comparative Stainless Steels 1 to 6.(Unit: mass %) Comparative Comparative Comparative ComparativeComparative Comparative Stainless Stainless Stainless StainlessStainless Stainless Steel 1 Steel 2 Steel 3 Steel 4 Steel 5 Steel 6 C0.16 0.01 0.01 0.01 0.01 0.01 Cr 12.11  10.48  13.58  12.00  11.98 11.88  Ni 9.51 9.44 9.47 7.03 11.51  9.53 Al 1.32 1.33 1.31 1.35 1.300.75 Mo 1.10 1.13 1.14 1.15 1.15 1.15 W 2.25 2.25 2.26 2.27 2.27 2.26 Ti— — — — — — Co — — — — — — Nb + V — — — — — — Si — — — — — — Mn — — — —— — N — — — — — — Fe + Balance Inevitable Impurities Mo + 0.5W 2.23 2.262.27 2.29 2.29 2.28 Mo/W 0.49 0.50 0.50 0.51 0.51 0.51 Note 1: Mark “—”means that the element was not intentionally added or the element wasbelow detection limit. Note 2: “Inevitable Impurities” are P, S, Sb, Snand As.

TABLE 4 Chemical Composition of Comparative Stainless Steels 7 to 12.(Unit: mass %) Comparative Comparative Comparative ComparativeComparative Comparative Stainless Stainless Stainless StainlessStainless Stainless Steel 7 Steel 8 Steel 9 Steel 10 Steel 11 Steel 12 C0.01 0.01 0.01 0.01 0.01 0.01 Cr 12.04  12.08  12.03  12.01  12.05 12.10  Ni 9.50 9.55 9.52 9.50 9.53 4.44 Al 2.02 1.28 1.33 1.34 1.25 1.27Mo 1.11 2.21 1.33 0.73 — 1.15 W 2.23 — 1.75 2.92 4.43 2.25 Ti — — — — —— Co — — — — — 5.03 Nb + V — — — — — — Si — — — — — — Mn — — — — — — N —— — — — — Fe + Balance Inevitable Impurities Mo + 0.5W 2.23 2.21 2.212.19 2.22 2.28 Mo/W 0.50 Mo only 0.75 0.25 W only 0.51 Note 1: Mark “—”means that the element was not intentionally added or the element wasbelow detection limit. Note 2: “Inevitable Impurities” are P, S, Sb, Snand As.

TABLE 5 Chemical Composition of Comparative Stainless Steels 13 to 17.(Unit: mass %) Comparative Comparative Comparative ComparativeComparative Stainless Stainless Stainless Stainless Stainless Steel 13Steel 14 Steel 15 Steel 16 Steel 17 C 0.01 0.01 0.01 0.01 0.01 Cr 11.97 11.88  12.08  12.01  12.04  Ni 9.47 9.52 9.53 9.44 9.50 Al 1.31 1.321.31 1.31 1.32 Mo 1.16 1.13 1.14 1.15 1.17 W 2.26 2.25 2.24 2.24 2.20 Ti— 0.51 — — — Co — — — — — Nb + V 1.0  — — — — Si — — 1.24 — — Mn — — —1.23 — N — — — — 0.15 Fe + Balance Inevitable Impurities Mo + 0.5W 2.292.26 2.26 2.27 2.27 Mo/W 0.51 0.50 0.51 0.51 0.53 Note 1: Mark “—” meansthat the element was not intentionally added or the element was belowdetection limit. Note 2: “Inevitable Impurities” are P, S, Sb, Sn andAs.

(Steel Property Measurement and Evaluation)

Each of the above samples (Inventive Stainless Steels 1 to 12 andComparative Stainless Steels 1 to 17) was observed or measured for: themicrotexture; the room temperature 0.02% proof stress and the roomtemperature tensile strength (as representatives of the mechanicalstrength); the percent elongation and the percent reduction in area (asrepresentatives of the ductility); the room temperature impact energyabsorption (as a representative of the toughness); and the pittingpotential (as a representative of the corrosion resistance). Theseobservation and measurement methods will be briefly explained below.

The microtexture of the starting sample bars was observed with anoptical microscope. When a starting sample bar has a martensite matrixin which the content of δ-ferrite phase precipitation does not exceed1.0% and the content of retained austenite phase precipitation does notexceed 10%, the sample bar is rated as “Pass” in terms of themicrotexture. A sample bar that does not satisfy the above criterion israted as “Fail”. The content of δ-ferrite phase precipitation wasmeasured according to the inclusion rating defined in JIS G 0555. Thecontent of retained austenite phase precipitation was measured by X raydiffraction method.

For the 0.02% proof stress and tensile strength measurements, theabove-described staring sample bars were further machined to form atensile test rod (gauge length of 30 mm; outer diameter of 6 mm). Eachtensile test rod was measured for the 0.02% proof stress and the tensilestrength at room temperature according to the test methods defined inJIS Z 2241. A tensile test rod having a 0.02% proof stress of 1000 MPaor more is rated as “Pass” in terms of the 0.02% proof stress; a tensiletest rod that does not satisfy this criterion is rated as “Fail”. Atensile test rod having a tensile strength of 1550 MPa or more is ratedas “Pass” in terms of the tensile strength; a tensile test rod that doesnot satisfy this criterion is rated as “Fail”. A tensile test rod havingan elongation of 10% or more is rated as “Pass” in terms of the percentelongation; a tensile test rod that does not satisfy this criterion israted as “Fail”. A tensile test rod having a reduction in area of 30% ormore is rated as “Pass” in terms of the percent reduction in area; atensile test rod that does not satisfy this criterion is rated as“Fail”.

For the impact energy absorption measurement, the above-describedstarting sample bars were further machined to form a Charpy test piecehaving a 2 mm V-notch. Each test piece was subjected to a Charpy impacttest at room temperature according to JIS Z 2242. A test piece having animpact energy absorption of 30 J or more is rated as “Pass” in terms ofthe impact energy absorption; a test piece that does not satisfy thiscriterion is rated as “Fail”.

For the pitting potential measurement, the above-described startingsample bars were further machined to form a flat-plate test piece (15 mmlong, 15 mm wide and 3 mm thick). Then, the entire surface of eachflat-plate test piece was insulation coated except for the measurementsurface (area of 1.0 cm²). The pitting potential measurement wasperformed in a 30° C., 3.0% aqueous NaCl solution at a sweep rate of 20mV/min. A flat-plate test piece having a pitting potential of 150 mV ormore is rated as “Pass” in terms of the pitting potential; a test piecethat does not satisfy this criterion is rated as “Fail”.

The observation and measurement results are summarized in Tables 6 to10.

TABLE 6 Observation and Measurement Results of Inventive StainlessSteels 1 to 6. Inventive Inventive Inventive Inventive InventiveInventive Stainless Stainless Stainless Stainless Stainless StainlessSteel 1 Steel 2 Steel 3 Steel 4 Steel 5 Steel 6 Microtexture Pass PassPass Pass Pass Pass 0.02% Proof Pass Pass Pass Pass Pass Pass StressTensile Pass Pass Pass Pass Pass Pass Strength Percent Pass Pass PassPass Pass Pass Elongation Percent Pass Pass Pass Pass Pass PassReduction in Area Impact Pass Pass Pass Pass Pass Pass Energy AbsorptionPitting Pass Pass Pass Pass Pass Pass Potential

TABLE 7 Observation and Measurement Results of Inventive StainlessSteels 7 to 12. Inventive Inventive Inventive Inventive InventiveInventive Stainless Stainless Stainless Stainless Stainless StainlessSteel 7 Steel 8 Steel 9 Steel 10 Steel 11 Steel 12 Microtexture PassPass Pass Pass Pass Pass 0.02% Proof Pass Pass Pass Pass Pass PassStress Tensile Pass Pass Pass Pass Pass Pass Strength Percent Pass PassPass Pass Pass Pass Elongation Percent Pass Pass Pass Pass Pass PassReduction in Area Impact Pass Pass Pass Pass Pass Pass Energy AbsorptionPitting Pass Pass Pass Pass Pass Pass Potential

TABLE 8 Observation and Measurement Results of Comparative StainlessSteels 1 to 6. Comparative Comparative Comparative ComparativeComparative Comparative Stainless Stainless Stainless StainlessStainless Stainless Steel 1 Steel 2 Steel 3 Steel 4 Steel 5 Steel 6Microtexture Fail Pass Fail Pass Fail Pass 0.02% Proof Fail Pass PassPass Fail Fail Stress Tensile Fail Pass Pass Pass Fail Fail StrengthPercent Pass Pass Fail Fail Pass Pass Elongation Percent Pass Pass FailFail Pass Pass Reduction in Area Impact Pass Pass Fail Fail Pass PassEnergy Absorption Pitting Fail Fail Pass Pass Pass Pass Potential

TABLE 9 Observation and Measurement Results of Comparative StainlessSteels 7 to 12. Comparative Comparative Comparative ComparativeComparative Comparative Stainless Stainless Stainless StainlessStainless Stainless Steel 7 Steel 8 Steel 9 Steel 10 Steel 11 Steel 12Microtexture Pass Pass Pass Pass Pass Fail 0.02% Proof Pass Fail FailPass Pass Fail Stress Tensile Pass Fail Fail Pass Pass Fail StrengthPercent Fail Pass Pass Pass Pass Fail Elongation Percent Fail Pass PassPass Pass Fail Reduction in Area Impact Fail Pass Pass Fail Fail FailEnergy Absorption Pitting Pass Pass Pass Pass Pass Pass Potential

TABLE 10 Observation and Measurement Results of Comparative StainlessSteels 13 to 17. Comparative Comparative Comparative ComparativeComparative Stainless Stainless Stainless Stainless Stainless Steel 13Steel 14 Steel 15 Steel 16 Steel 17 Microtexture Pass Fail Fail FailFail 0.02% Pass Pass Fail Fail Fail Proof Stress Tensile Pass Pass FailFail Fail Strength Percent Fail Pass Fail Pass Pass Elongation PercentFail Pass Fail Pass Pass Reduction in Area Impact Energy Fail Fail FailPass Pass Absorption Pitting Pass Pass Pass Pass Pass Potential

As shown in Tables 6 and 7, Inventive Stainless Steels 1 to 12 have amartensite matrix containing only small amounts of δ-ferrite phaseprecipitates and retained austenite phase precipitates. In addition, forall of Inventive Stainless Steels 1 to 12, fine β-NiAl phaseprecipitates of 10 nm or smaller in particle size are uniformlydispersed in each martensite grain. Also, all of Inventive Steels arerated as “Pass” in terms of: the mechanical properties including themechanical strength (0.02% proof stress and tensile strength); theductility (the percent elongation and the percent reduction in area);and the toughness (the impact energy absorption). Further, all ofInventive Steels have a good corrosion resistance (a high pittingpotential). It is thus demonstrated from the above results that theprecipitation-hardening martensitic stainless steel of the inventionhas, compared with conventional ones, a far better balance among a highmechanical strength, a high ductility, a high toughness and a highcorrosion resistance.

By contrast, Comparative Stainless Steels 1 to 17 are rated as “Fail” interms of at least one of the micro texture, mechanical strength,ductility, toughness and corrosion resistance. That is, ComparativeStainless Steels do not satisfy all of these performances. Specifically,Comparative Stainless Steel 1 is rated as “Fail” in terms of themicrotexture, mechanical strength and corrosion resistance, because theC content falls out of the invention's specification range.

Comparative Stainless Steel 2 is rated as “Fail” in terms of thecorrosion resistance, because the Cr content falls below the invention'sspecification. Comparative Stainless Steel 3 is rated as “Fail” in termsof the microtexture, ductility and toughness, because the Cr contentexceeds the invention's specification.

Comparative Stainless Steel 4 is rated as “Fail” in terms of theductility and toughness, because the Ni content falls below theinvention's specification. Comparative Stainless Steel 5 is rated as“Fail” in terms of the microtexture and mechanical strength, because theNi content exceeds the invention's specification.

Comparative Stainless Steel 6 is rated as “Fail” in terms of themechanical strength, because the Al content falls below the invention'sspecification. Comparative Stainless Steel 7 is rated as “Fail” in termsof the ductility and toughness, because the Al content exceeds theinvention's specification.

Comparative Stainless Steel 12 is rated as “Fail” in terms of themicrotexture, mechanical strength, ductility and toughness, because theamount of substitution of Ni by Co exceeds the invention'sspecification. Comparative Stainless Steel 13 is rated as “Fail” interms of the ductility and toughness, because the total content of Nband V exceeds the invention's specification. Comparative Stainless Steel14 is rated as “Fail” in terms of the microtexture and toughness,because the Ti content exceeds the invention's specification.Comparative Stainless Steel 15 is rated as “Fail” in terms of themicrotexture, mechanical strength, ductility and toughness, because theSi content exceeds the invention's specification. Comparative StainlessSteel 16 is rated as “Fail” in terms of the microtexture and mechanicalstrength, because the Mn content exceeds the invention's specification.Comparative Stainless Steel 17 is rated as “Fail” in terms of themicrotexture and mechanical strength, because the N content exceeds theinvention's specification.

Next, the invention's preferred compositional balance between the Mo andthe W will be discussed by comparing the measurement results ofInventive Stainless Steel 1 and Comparative Stainless Steels 8 to 11.FIG. 4 is a graph showing a relationship between the “[Mo content]/[Wcontent]” ratio and the tensile strength. FIG. 5 is a graph showing arelationship between the “[Mo content]/[W content]” ratio and the impactenergy absorption.

FIG. 4 shows that as the “[Mo content]/[W content]” ratio decreases(i.e., the W content increases relative to the Mo content), the tensilestrength increases. This is probably due to the positive effect of solidsolution strengthening by the W and formation of Laves (Fe₂W) phase.Microtexture observation of Comparative Stainless Steel 11 showed thatLaves phase was actually formed and precipitated.

As for the impact energy absorption, as shown in FIG. 5, as the “[Mocontent]/[W content]” ratio decreases (i.e., the W content increasesrelative to the Mo content), the impact energy absorption decreases. Inthis case, however, an excessive formation of Laves phase probably hasan adverse effect of significantly degrading the toughness of the W-richstainless steels. The results shown in FIGS. 4 and 5 confirm that “[Mocontent]/[W content]” ratios from 0.4 to 0.6 are desirable in order toachieve an excellent balance between a high mechanical strength and ahigh toughness.

(Steam Turbine Long Blade)

A 51-inch steam turbine long blade (see FIG. 1) was formed fromInvention Stainless Steel 1 as follows: First, Inventive Stainless Steel1 was subjected to a vacuum carbon deoxidation, which involves meltingand deoxidizing the steel in a high vacuum of 5.0×10⁻³ Pa by utilizingthe chemical reaction of “C+O→CO”. Next, the deoxidized steel was formedinto an electrode rod by extend forging. Then, the electrode rod wassubjected to an electroslag remelting, which involves immersing the rodin a molten slag, melting it by the Joule's heat generated by currentflow through it, and resolidifying it in a water cooled mold. By thiselectroslag remelting, a high-quality steel ingot was obtained.

The steel ingot was hot-forged, and then closed-die forged into a51-inch long blade. The die-formed long blade was solution heat treatedby maintaining it at 900° C. for 2 hours and quenching it by forcedcooling using a blower, and was then formed into a final shape bycutting. Next, the final shaped blade was aging heat treated bymaintaining it at 538° C. for 4 hours and cooling it in air. Finally,finish processing (such as straightening and surface polishing) wasperformed to complete the formation of the 51-inch long blade.

A test specimen was cut out from each of an end portion, a centerportion and a root portion of the thus formed steam turbine long bladein such a manner that the length direction of the test specimen wasparallel to the length direction of the blade. Then, each test specimenwas subjected to the above-described observation and measurements. Allof the blade portions are rated as “Pass” in terms of all of themicrotexture; the mechanical strength (the 0.02% proof stress and thetensile strength); the ductility (the percent elongation and the percentreduction in area); the toughness (the impact energy absorption); andthe corrosion resistance (the pitting potential).

As has been described, the precipitation-hardening martensitic stainlesssteel of the present invention has a homogeneous martensite matrixthroughout which intermetallic compound precipitates are uniformlydispersed. Also, the invention's stainless steel has a far betterbalance between a high mechanical strength and a high corrosionresistance than conventional stainless steels. Thus, the invention'sstainless steel can be advantageously applied tolonger-than-conventional steam turbine long blades.

Also, there can be provided: a high performance turbine rotor includingsuch a longer-than-conventional steam turbine long blade; a highperformance steam turbine including such a high performance turbinerotor; and a high performance thermal power plant including such a highperformance steam turbine. Further, the precipitation-hardeningmartensitic stainless steel of the invention can be used for components(such as blades) of other type turbines such as gas turbine compressors.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A precipitation-hardening martensitic stainlesssteel throughout which precipitates of intermetallic compounds aredispersed, the martensitic stainless steel comprising: 0.1 mass % orless of C; 11 to 13 mass % of Cr; 7.5 to 11 mass % of Ni; 0.9 to 1.7mass % of Al; 0.85 to 1.35 mass % of Mo; 1.75 to 2.75 mass % of W; andthe balance including Fe and inevitable impurities, wherein “[content ofthe Mo]+0.5×[content of the W]” is from 1.9 mass % to 2.5 mass %, and“[content of the Mo]/[content of the W]” is from 0.4 to 0.6.
 2. Theprecipitation-hardening martensitic stainless steel according to claim1, further including 0.4 mass % or less of Ti.
 3. Theprecipitation-hardening martensitic stainless steel according to claim1, wherein part of the Ni is substituted by 3 mass % or less of Co. 4.The precipitation-hardening martensitic stainless steel according toclaim 1, further including one or both of Nb and V in total amount of0.5 mass % or less.
 5. The precipitation-hardening martensitic stainlesssteel according to claim 1, further including 0.1 mass % or less of Siand/or 1 mass % or less of Mn.
 6. The precipitation-hardeningmartensitic stainless steel according to claim 1, wherein the inevitableimpurities include one or more of 0.5 mass % or less of P, 0.5 mass % orless of S, 0.1 mass % or less of Sb, 0.1 mass % or less of Sn, 0.1 mass% or less of As, and 0.1 mass % or less of N.
 7. Theprecipitation-hardening martensitic stainless steel according to claim1, wherein one of the intermetallic compounds is β-NiAl phase.
 8. Theprecipitation-hardening martensitic stainless steel according to claim1, wherein the precipitation-hardening martensitic stainless steel issolution heat treated at 850 to 950° C. followed by aging heat treatmentat 450 to 650° C.
 9. A turbine component formed of theprecipitation-hardening martensitic stainless steel according toclaim
 1. 10. A turbine rotor including the turbine component accordingto claim 9, wherein the turbine component is a steam turbine long blade.11. A steam turbine including the turbine rotor according to claim 10.12. A thermal power plant including the steam turbine according to claim11.
 13. A turbine component formed of the precipitation-hardeningmartensitic stainless steel according to claim
 2. 14. A turbinecomponent formed of the precipitation-hardening martensitic stainlesssteel according to claim
 3. 15. A turbine component formed of theprecipitation-hardening martensitic stainless steel according to claim4.
 16. A turbine component formed of the precipitation-hardeningmartensitic stainless steel according to claim 5.